Multi-carrier transmission systems which provide high speed data communication over a local subscriber loop connecting a customer to a central office are commonly referred to as “xDSL” systems, where “x” specifies a particular variant of DSL (digital subscriber line). The term xDSL refers to DSL technologies such as ADSL (asymmetric DSL), HDSL (high bit rate DSL), IDSL (ISDN DSL), SDSL (symmetric DSL), VDSL (very high speed DSL), etc. These and other types of xDSL systems are generically referred to herein as “DSL” systems.
In a DSL system, each customer has a modem for communicating with a digital subscriber line access multiplexer (DSLAM) at the central office of the service provider. The DSLAM terminates and aggregates the DSL circuits, handing them off onto other networking transports. Each communication channel between a customer and the central office is terminated by a pair of transceivers which communicate with each other. The total bandwidth of the channel interconnecting the customer and the central office is typically divided into several different sub-carriers. Each sub-carrier is centered at a particular frequency and has a particular bandwidth. One group of the sub-carriers is allocated for transmissions from the central office to the customer modem, i.e. the downstream direction. A second group of the sub-carriers is allocated for transmissions from the customer modem to the central office, i.e. the upstream direction. Additional sub-carriers can be allocated for overhead and control functions.
Data to be communicated between a customer modem and the central office is split into groups of bits, one group of bits per sub-carrier. Each group of bits is modulated onto a carrier, e.g. using quadrature amplitude modulation (QAM) and mapped into a vector defined by a point on the modulation “constellation.” The constellation specifies the allowable data points for transmission. Each point on the constellation is typically referred to as a symbol. The number of bits which is modulated on each subcarrier is referred to as the bit loading for this subcarrier. A symbol can represent more bits when a higher-order modulation scheme is used or fewer bits when a lower-order modulation scheme is used. During a symbol transmission time period, a symbol is transmitted on each sub-carrier in parallel with the other sub-carriers so that large amounts of data can be transmitted during each symbol period.
Conventional DSL equipment provides almost constant data rate for the duration of the link independent of the bandwidth required by the customer. However, most customers require high bandwidth only for a few hours per day. During the remainder of the time, the customer applications may require only a fraction of the usable bandwidth or possibly even no bandwidth at all. For example, voice applications such as VoIP typically require a bandwidth of 128 kbps. Yet, customers who have signed up only for a voice service have DSL equipment running for 24 hours a day without ever using the provided data rate which can range from 256 kbps to 3 Mbps or even higher depending on the type of DSL service. Maintaining a constant data rate during periods of low or no bandwidth demand unnecessarily wastes power. In addition, existing DSL lines expanded for triple play services (high-speed Internet, TV and voice) are usually always powered on. The result is an enormous demand of energy for telecommunication equipment, making telecommunication service providers some of the single largest energy consumers in the world.
The ADSL2 standard defines a low power mode (L2 mode). The L2 mode allows modems to reduce the bitloading and/or reduce the transmit power when no data or only a small amount of data is to be transmitted. In practice, the ADSL2 L2 mode is not widely used. A significant amount of fluctuating crosstalk can occur in a cable binder (i.e., bundle) when modems move back and forth between the L2 mode and regular full data transmission. This fluctuating crosstalk must be accounted for by all modems coupled to the same cable binder. Otherwise, data errors can occur. The concept of virtual noise has been introduced to control the non-stationary crosstalk caused by entering and exiting the ADSL2 L2 mode. With virtual noise, modems can be made aware of the DSL lines that are not active but which can potentially be activated. The modems can use this information to employ frequency-specific margins for providing protection against non-stationary crosstalk. However, conventional virtual noise techniques cause a significant loss of data rate when applied over the complete transmission band. In addition, the ADSL2 L2 mode does not allow for fully powering down the sub-carriers, limiting the power saving potential of the ADSL2 L2 mode. Furthermore, the ADSL2 L2 mode is only defined for downstream transmissions. As such, no power reduction can be realized at the customer side.